Magnetic-disk glass substrate and magnetic disk

ABSTRACT

A magnetic-disk glass substrate according to the present invention is a doughnut-shaped magnetic-disk glass substrate having a circular hole provided in the center, a pair of main surfaces, and an outer circumferential end surface and an inner circumferential end surface each including a side wall surface and a chamfered surface that is formed between each main surface and the side wall surface. A measurement point is provided on the outer circumferential end surface every 30 degrees in the circumferential direction with reference to a center of the glass substrate, and when a curvature radius of a shape of a portion between the side wall surface and the chamfered surface is determined at each measurement point, the difference in the curvature radius between neighboring measurement points is 0.01 mm or less.

CROSS-REFERENCE TO RELATED APPLICATIONS

This U.S. National stage application of International Patent ApplicationNo. PCT/JP2013/076614, filed Sep. 30, 2013, which, in turn, claimspriority under 35 U.S.C. §119(a) to Japanese Patent Application No.2012-218706, filed in Japan on Sep. 28, 2012, the entire contents ofwhich are hereby incorporated herein by reference.

BACKGROUND

Field of the Invention

The present invention relates to a magnetic-disk glass substrate and amagnetic disk.

Background Information

Nowadays, personal computers, digital versatile disc (DVD) recorders,and the like have a built-in hard disk drive (HDD) for data recording.In particular, a magnetic disk in which a magnetic layer is provided ona glass substrate is used in a hard disk drive that is used in a devicepremised on portability, such as a notebook-type personal computer, andmagnetic recording information is recorded on or read from the magneticlayer with a magnetic head that flies slightly above the surface of themagnetic disk. At this time, the rotation rate of the magnetic disk isabout 5400 rpm, for example. A glass substrate having a higher rigidityand a higher impact resistance than a metal substrate (aluminumsubstrate) or the like has is preferably used as a substrate of themagnetic disk. It should be noted that the magnetic-disk glass substrategenerally has a thickness of 0.635 mm or 0.8 mm in a 2.5-inch magneticdisk, for example.

In recent years, there have been calls for a magnetic-disk for use in astationary server device such as a network server device that is rotatedat a higher rotation speed in order to further improve the access timeand the transfer rate. In the case where a magnetic disk is rotated at ahigher rotation speed than previously, a conventional magnetic- diskaluminum substrate does not have enough rigidity even if the thicknessis increased, and therefore, fluttering occurs. Here, fluttering is aphenomenon in which a substrate vibrates (flutters) due to the rotationof the substrate. Therefore, JP 2008-226376A discloses a magnetic-diskglass substrate in which ⊕S that is a difference between a maximum valueSmax and minimum value Smin of the gap S between the inner circumferenceand the outer circumference of a projection image projected on a planeparallel with a main surface is set to less than 2 μm in order to reducefluttering at a high rotation speed.

SUMMARY

In a conventional magnetic-disk glass substrate, the difference betweenthe maximum value and the minimum value of the gap between the innercircumference and the outer circumference of a main surface isprescribed. However, it was confirmed that when a magnetic disk isrotated at a high rotation speed, fluttering sometimes occurs due to theshape of the end surface of the glass substrate.

Therefore, it is an object of the present invention to provide amagnetic-disk glass substrate and a magnetic disk capable of furtherreducing fluttering at a high rotation speed.

As a result of intensive research by the inventors of the presentinvention to further reduce fluttering at a high rotation speed, it wasfound that it is possible to further reduce fluttering by reducing thechange in the shape of the outer circumferential end surface (includinga side wall surface orthogonal to main surfaces and chamfered surfacesbetween the main surfaces and the side wall surface) of the glasssubstrate in the circumferential direction.

The inventors of the present invention consider the reason for this tobe as follows. That is, it is thought that when there is a large changein the shape of the outer circumferential end surface in thecircumferential direction of the glass substrate during a short cycle,the amount of air that comes into contact with the outer circumferentialend surface changes significantly, and thus the airflow around the outercircumferential end surface caused by the rotation of the substrate isdestabilized. Moreover, some recent magnetic-disk drive devices areprovided with a wall (shroud) for covering the outer circumference ofthe magnetic disk that is attached to a spindle, and by reducing the gapbetween the wall and the outer circumferential end surface of themagnetic disk, it is possible to stabilize the airflow. However, it isthought that when the change in the shape of the outer circumferentialend surface in the circumferential direction of the glass substratebecomes large in a state in which the gap is small, the rate of thechange in the gap between the wall and the outer circumferential endsurface of the magnetic disk increases, and thus the airflow in the gapis likely to be disturbed. Accordingly, by reducing the change in theshape of the outer circumferential end surface in the circumferentialdirection of the glass substrate, it is possible to stabilize theairflow around the outer circumferential end surface of the glasssubstrate and to keep the gap between the wall and the outercircumferential end surface of the magnetic disk substantially constant,thus making it possible to further reduce fluttering.

Therefore, a first aspect of the present invention is a doughnut-shapedmagnetic-disk glass substrate having a circular hole provided in thecenter, a pair of main surfaces, and an outer circumferential endsurface and an inner circumferential end surface each including a sidewall surface and a chamfered surface that is formed between each mainsurface and the side wall surface, wherein a measurement point isprovided on the outer circumferential end surface every 30 degrees inthe circumferential direction with reference to a center of the glasssubstrate, and when a curvature radius of a shape of a portion betweenthe side wall surface and the chamfered surface is determined at eachmeasurement point, a difference in the curvature radius betweenneighboring measurement points is 0.01 mm or less.

In the magnetic-disk glass substrate, it is preferable that thedifference in the curvature radius between neighboring measurementpoints is 0.005 mm or less.

In the magnetic-disk glass substrate, it is preferable that ameasurement point is provided every 30 degrees in the circumferentialdirection with reference to the center of the glass substrate, and whena curvature radius of a shape of a portion between the main surface andthe chamfered surface is determined at each measurement point as asecond curvature radius, a difference in the second curvature radiusbetween neighboring measurement points is 0.004 mm or less.

In the magnetic-disk glass substrate, it is preferable that the glasssubstrate has a thickness of 0.635 mm or less.

A second aspect of the present invention is a magnetic disk in which atleast a magnetic layer is formed on the above-described magnetic-diskglass substrate.

With a magnetic-disk glass substrate and a magnetic disk according tothe present invention, it is possible to further reduce fluttering at ahigh rotation speed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view of a magnetic-disk glass substrate according toan embodiment;

FIG. 1B is an enlarged cross-sectional view taken along line X-X shownin FIG. 1A;

FIG. 1C is a further enlarged view of a portion of FIG. 1B;

FIG. 2 is an enlarged view of main portions of FIG. 1B;

FIG. 3A is a drawing illustrating a method for polishing a glasssubstrate according to an embodiment;

FIG. 3B is a drawing illustrating the method for polishing a glasssubstrate according to the embodiment;

FIG. 3C is a drawing illustrating the method for polishing a glasssubstrate according to the embodiment; and

FIG. 4 is a drawing illustrating the method for polishing a glasssubstrate according to the embodiment.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a magnetic-disk glass substrate and a magnetic disk of thisembodiment will be described in detail.

[Magnetic-Disk Glass Substrate]

Aluminosilicate glass, soda-lime glass, borosilicate glass, or the likecan be used as a material for a magnetic-disk glass substrate of thisembodiment. In particular, aluminosilicate glass can be preferably usedbecause it can be chemically strengthened and be used to produce amagnetic-disk glass substrate having excellent flatness of its mainsurfaces and excellent strength of the substrate. Amorphousaluminosilicate glass is more preferable.

Although there is no limitation on the composition of the magnetic-diskglass substrate of this embodiment, the glass substrate of thisembodiment is amorphous aluminosilicate glass that preferably contains,in terms of oxide amount in mol %, SiO₂ in an amount of 50 to 75%, Al₂O₃in an amount of 1 to 15%, at least one component selected from Li₂O,Na₂O and K₂O in a total amount of 5 to 35%, at least one componentselected from MgO, CaO, SrO, BaO and ZnO in a total amount of 0 to 20%,and at least one component selected from ZrO₂, TiO₂, La₂O₃, Y₂O₃, Ta₂O₅,Nb₂O₅ and HfO₂ in a total amount of 0 to 10% (this composition isreferred to as “glass composition 1” hereinafter).

Also, the glass substrate of this embodiment may be amorphousaluminosilicate glass that preferably contains, as disclosed in JP2009-99239A for example, in mass %, SiO₂ in an amount of 57 to 75%,Al₂O₃ in an amount of 5 to 20% (it should be noted that the total amountof SiO₂ and Al₂O₃ is 74% or more), ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃and TiO₂ in a total amount of more than 0% to 6% or less, Li₂O in anamount of more than 1% to 9% or less, Na₂O in an amount of 5 to 18% (itshould be noted that a mass ratio Li₂O/Na₂O is 0.5 or less), K₂O in anamount of 0 to 6%, MgO in an amount of 0 to 4%, CaO in an amount of morethan 0% to 5% or less (it should be noted that the total amount of MgOand CaO is 5% or less and the content of CaO is larger than that ofMgO), and SrO+BaO in an amount of 0 to 3%.

As disclosed in Japanese Patent No. 4815002, for example, the glasssubstrate of this embodiment may be crystallized glass that contains, interms of oxide amount in mass %, SiO₂ in an amount of 45.60 to 60%,Al₂O₃ in an amount of 7 to 20%, B₂O₃ in an amount of 1.00 to 8% or less,P₂O₅ in an amount of 0.50 to 7%, TiO₂ in an amount of 1 to 15%, and RO(it should be noted that R represents Zn and Mg) in a total amount of 5to 35%, CaO in an amount of 3.00% or less, BaO in an amount of 4% orless, no PbO component, no As₂O₃ component, no Sb₂O₃ component, no Cl⁻component, no NO⁻ component, no SO²⁻ component, no F⁻ component, and oneor more selected from RAl₂O₄ and R₂TiO₄ (it should be noted that Rrepresents one or more selected from Zn and Mg) as a main crystal phase,and in which the particle size of crystals in the main crystal phase iswithin a range of 0.5 nm to 20 nm, the degree of crystallization is 15%or less, and the specific gravity is 2.95 or less.

The composition of the magnetic-disk glass substrate of this embodimentmay include SiO₂, Li₂O and Na₂O, and one or more alkaline earth metaloxides selected from the group consisting of MgO, CaO, SrO and BaO asessential components, the molar ratio of the content of CaO to the totalcontent of MgO, CaO, SrO and BaO (CaO/(MgO+CaO+SrO+BaO)) may be 0.20 orless, and the glass-transition temperature may be 650° C. or higher. Themagnetic-disk glass substrate having such a composition is preferablyfor a magnetic-disk glass substrate to be used in a magnetic disk forenergy-assisted magnetic recording (this composition is referred to as“glass composition 2” hereinafter).

FIGS. 1A to 1C and FIG. 2 show the shape of a magnetic-disk glasssubstrate G of this embodiment. FIG. 1A is a plan view of themagnetic-disk glass substrate of this embodiment. FIG. 1B is an enlargedcross-sectional view taken along line X-X shown in FIG. 1A. FIG. 1C is afurther enlarged view of a portion of FIG. 1B. FIG. 2 is an enlargedview of main portions of FIG. 1B. Here, it should be noted that there isno need to actually cut the substrate in order to check the shape of thecross section, and it is sufficient to measure the shape of the crosssection using a contour shape measuring machine. Specifically, forexample, it is sufficient to fix the glass substrate such that thechamfered surface on the outer circumferential side is substantiallyhorizontal, and to move the fixed glass substrate or a stylus of thecontour shape measuring machine in the radial direction of the glasssubstrate.

As shown in FIG. 1A, the magnetic-disk glass substrate G of thisembodiment has a doughnut shape that is provided with a circular innerhole in the center thereof and has an annular contour. As shown in FIG.1B, the magnetic-disk glass substrate G of this embodiment has a pair ofmain surfaces 11 p and 12 p and two side wall surfaces, which are a sidewall surface on the inner circumferential side (that is, a side wallsurface of the circular hole) and a side wall surface on the outercircumferential side (that is, a side wall surface of the contour). InFIG. 1B, the side wall surface 11 w of the glass substrate G is the sidewall surface on the outer circumferential side. It is preferable thatthe side wall surface 11 w on the outer circumferential side of theglass substrate G includes a surface orthogonal to each of the two ofmain surfaces 11 p and 12 p. Chamfered surfaces 11 c and 12 c arerespectively formed between the pair of main surfaces 11 p and 12 p andthe side wall surface 11 w on the outer circumferential side. Here,although FIG. 1B shows the case where the chamfered surfaces 11 c and 12c are formed in a plane shape as an example, the chamfered surfaces 11 cand 12 c may be formed so as to include a surface curved outward fromthe glass substrate G. In this case, it is preferable that the degree ofthe curvature is substantially constant in the circumferential directionof the glass substrate G.

The shape of the end portion on the outer circumferential side of themagnetic-disk glass substrate G of this embodiment will be describedwith reference to FIG. 1C. In FIG. 1C, the linear portions of the mainsurface 11 p, the side wall surface 11 w, and the chamfered surface 11 care expressed as Lp, Lw, and Lc, respectively. In addition, in FIG. 1C,the intersection of the linear portion Lp of the main surface 11 p andthe linear portion Lc of the chamfered surface 11 c is given as P11, theintersection of the linear portion Lw of the side wall surface 11 w andthe linear portion Lc of the chamfered surface 11 c is given as P12, andthe intersection of the linear portion Lp of the main surface 11 p andthe linear portion Lw of the side wall surface 11 w is given as P13.

It is preferable that the angle formed by the linear portion Lp of themain surface 11 p and the linear portion Lc of the chamfered surface 11c (chamfering angle: θ1) is, for example, 40 to 70 degrees. It ispreferable that the angle formed by the linear portion Lw of the linearportion surface 11 w and the linear portion Lc of the chamfered surface11c (chamfering angle: θ2) is, for example, 20 to 50 degrees. Also, itis preferable that a distance D1 between the point P11 and the point P13is set to 0.05 to 0.20 mm and a distance D2 between the point P12 andthe point P13 is set to 0.10 to 0.30 mm. By forming the shape within theabove-described ranges, it is possible to prevent problems such aschipping of the outer circumferential end portion and dropping of thesubstrate, even when the outer circumferential end portion is held in aninspection step, a film forming step, a HDD assembling step, and thelike after finishing the manufacture of the glass substrate.

It should be noted that while the end portion near one main surface onthe outer circumferential side of the magnetic-disk glass substrate hasbeen described with reference to FIG. 1C, other end portions (that is,the end portion near the other main surface on the outer circumferentialside and the end portions on the inner circumferential side) are formedin the same manner.

In the description below, the side wall surface 11 w and the chamferedsurfaces 11 c and 12 c are collectively referred to as the “outercircumferential end surface”, and the side wall surface and thechamfered surfaces on the inner circumferential side (not shown) arecollectively referred to as the “inner circumferential end surface”.

Next, a method for determining the curvature radius of the shape of theportion between the side wall surface 11 w and the chamfered surface 11c will be described with reference to FIG. 2. In FIG. 2, R denotes theradius of a circle C2 forming the curvature of the shape of the portionbetween the side wall surface 11 w and the chamfered surface 11 c, andthe curvature radius of the shape of that portion. It is possible todetermine the curvature radius R as follows, for example. First, theintersection of a virtual line L1 obtained by extending the linearportion of the chamfered surface 11 c and a virtual line L2 obtained byextending the linear portion of the side wall surface 11 w is given asP1. Next, a virtual line L3 is set that passes through the intersectionP1 and extends orthogonally with respect to the linear portion of thechamfered surface 11 c. Next, the intersection of the portion betweenthe side wall surface 11 w and the chamfered surface 11 c and thevirtual line L3 is given as P2. Moreover, a circle C1 around theintersection P2 that has a predetermined radius (50 μm, for example) isset on the cross section of the glass substrate G. In addition, twointersections of the portion between the side wall surface 11 w and thechamfered surface 11 c and the circumference of the circle C1 are givenas P3 and P4, respectively. Furthermore, a circle C2 is set that passesthrough the three intersections P2, P3, and P4.

By determining the radius of the circle C2, it is possible to determinethe curvature radius R of the shape of the portion between the side wallsurface 11 w and the chamfered surface 11 c.

It should be noted that the curvature radius of the shape of the portionbetween the side wall surface 11 w and the chamfered surface 12 c can bedetermined in the same manner as described above.

In this embodiment, measurement points are provided every 30 degrees inthe circumferential direction with reference to a center C of the glasssubstrate G (see FIG. 1A). That is, there are twelve measurement points.When the curvature radius R of the shape of the portion between the sidewall surface 11 w and the chamfered surface 11 c is determined at eachmeasurement point, the twelve differences (absolute values) in total(twenty-four differences when including the shapes of the portionsbetween the side wall surface 11 w and the other chamfered surface ofthe disk) of the curvature radius R between the neighboring measurementpoints are each set to 0.01 mm or less. Thus, it is possible to reducethe change in the shape of the outer circumferential end surface in thecircumferential direction of the glass substrate G, and therefore, inthe case where a magnetic disk produced using this glass substrate G isrotated at a high rotation speed, it is possible to stabilize theairflow around the outer circumferential end surface of the magneticdisk. Moreover, in the case where this magnetic disk is attached to aspindle of a magnetic-disk drive device, it is possible to keep the gapbetween a wall (shroud) that covers the outer circumference of themagnetic disk and the outer circumferential end surface of the magneticdisk substantially constant, and therefore, it is possible to stabilizethe airflow in the gap. Accordingly, with the glass substrate G of thisembodiment, it is possible to further reduce fluttering at a highrotation speed. It should be noted that a difference of the curvatureradius R between the neighboring measurement points of 0.005 mm or lessis preferable because it is possible to further reduce fluttering at ahigh rotation speed.

It should be noted that the curvature radius of the shape of the portionbetween the main surface 11 p and the chamfered surface 11 c (secondcurvature radius) may be determined in the same manner as describedabove. Specifically, the intersection of the virtual line L1 obtained byextending the linear portion of the chamfered surface 11 c and a virtualline L4 (not shown) obtained by extending the linear portion of the mainsurface 11 p is given as P5 (not shown). Next, a virtual line L5 (notshown) is set that passes through the intersection P5 and extendsorthogonally with respect to the main surface 11 p. Next, theintersection of the portion between the main surface 11 p and thechamfered surface 11 c and the virtual line L5 is given as P6 (notshown). Moreover, a circle C3 (not shown) around the intersection P6that has a predetermined radius (10 μm, for example) is set on the crosssection of the glass substrate G. In addition, two intersections of theportion between the main surface 11 p and the chamfered surface 11 c andthe circumference of the circle C3 are given as P7 and P8 (not shown),respectively. Furthermore, a circle C4 (not shown) is set that passesthrough the three intersections P6, P7, and P8.

By determining the radius of the circle C4, it is possible to determinethe second curvature radius of the shape of the portion between the mainsurface 11 p and the chamfered surface 11 c.

It should be noted that the second curvature radius of the shape of theportion between the main surface 12 p and the chamfered surface 12 c canbe determined in the same manner as described above.

For example, when measurement points are provided every 30 degrees inthe circumferential direction with reference to the center C of theglass substrate G and the second curvature radius is determined at eachmeasurement point (twelve measurement points), the difference of thesecond curvature radius between the neighboring measurement points maybe set to 0.004 mm or less. Thus, it is possible to further reduce thechange in the shape of the outer circumferential end surface (includingthe portion between the main surface and the chamfered surface) in thecircumferential direction of the glass substrate G, and therefore, inthe case where a magnetic disk produced using this glass substrate G isrotated at a high rotation rate, it is possible to stabilize the airflowaround the outer circumferential end surface of the magnetic disk.

Although there is no limitation on the size of the magnetic-disk glasssubstrate G of this embodiment, the magnetic-disk glass substrate G mayhave a nominal diameter of 2.5 inches, for example. The magnetic-diskglass substrate G of this embodiment may be used for, for example, amagnetic disk to be integrated in a magnetic-disk drive device to bemounted in a server device, a notebook-type personal computer, or thelike.

Incidentally, in order for a conventional magnetic disk to be rotated ata high rotation speed of, for example, 10000 rpm or more, it was soughtto ensure a desired thickness at which no fluttering would occur whenthe magnetic disk was rotated at such a high rotation speed. On theother hand, in recent years, the size and the thickness of magnetic-diskdrive devices have been reduced based on demands for the reduction ofthe size and the thickness of notebook-type personal computers and thelike. As a result, demands for a reduction of the thickness of magneticdisks have increased. Here, in the case where the thickness of amagnetic disk is merely reduced (thinned), a conventional magnetic diskis likely to be affected by the airflow caused by the rotation of themagnetic disk, thus making it difficult to reduce fluttering. Therefore,it is difficult to meet the demands for the reduction of the thicknessof the conventional magnetic disk. On the other hand, in thisembodiment, it is possible to stabilize the airflow around the outercircumferential end surface of the magnetic disk by reducing the changein the shape of the outer circumferential end surface in thecircumferential direction of the glass substrate G, and therefore, it ispossible to reduce fluttering even if the glass substrate G is thin. Forexample, it is preferable that the glass substrate G has a thickness T(see FIG. 1B) of nominally 0.635 mm or less because a significant effectcan be obtained by using the structure of this embodiment.

It should be noted that in the case where a glass substrate has athickness of “nominally 0.635 mm”, the case where the actual thicknessis slightly larger or slightly smaller than 0.635 mm is included.

[Method for Manufacturing Magnetic-Disk Glass Substrate]

Hereinafter, a method for manufacturing the magnetic-disk glasssubstrate of this embodiment will be described for each step. It shouldbe noted that the order of the steps may be changed as appropriate.

(1) Raw Glass Plate Molding and Rough Grinding Step

After forming a sheet of plate glass by, for example, a float method, araw glass plate having a predetermined shape from which a magnetic-diskglass substrate is to be made is cut out from this sheet of plate glass.A raw glass plate may also be molded by, for example, pressing using anupper mold and a lower mold instead of the float method. It should benoted that the method for manufacturing a raw glass plate is not limitedto these methods and a raw glass plate can also be manufactured by aknown manufacturing method such as a down draw method, a redraw methodor a fusion method.

It should be noted that rough grinding processing using loose abrasiveparticles may be performed on two main surfaces of the raw glass plateas needed.

(2) Circular Hole Forming Step

A circular hole is formed in the center of the raw glass plate using acylindrical drill, and thus an annular raw glass plate is obtained. Itshould be noted that a circular hole may be formed by forming a circularcutting line on the surface of the raw glass plate using a diamondcutter or the like and by cutting the raw glass plate along the cuttingline.

(3) Chamfering Step

After the circular hole forming step, a chamfering step of formingchamfered surfaces at the end portions (outer circumferential endportion and inner circumferential end portion) is performed. Thechamfering step can be performed using a conventionally known device andmethod, and may be performed using, for example, a rotating formedgrindstone while supplying a grinding liquid to a portion subjected togrinding processing. A groove may be formed in advance on the surface ofthe formed grindstone so as to obtain the end portions having desiredshapes after the processing. In the chamfering step, first, roughgrinding is performed on the outer circumferential end portion and theinner circumferential end portion of the annular raw glass plate using arelatively rough-grit diamond grindstone or the like, and thus chamferedshapes are formed at a relatively high speed. Next, final grinding isperformed on the chamfered surfaces using a grindstone that has a lowpolishing rate and does not damage the surfaces of the end portions,such as a relatively fine-grit diamond grindstone, so as to have surfaceproperties approaching a mirror surface. At this time, grindingprocessing is performed by rotating both the raw glass plate and thegrindstone and bringing them into contact with each other, thus formingthe shape of the raw glass plate in the circumferential direction.

Here, the surface roughness after the finishing can be made small byincreasing the grit of the grindstone to be used in grinding processing(that is, by reducing the particle diameters of diamond abrasiveparticles), thus making it possible to reduce machining allowance insubsequent end surface polishing. As the machining allowance in the endsurface polishing is smaller, the shape formed in the grinding step isretained, thus making it possible to enhance the shape accuracy. Thatis, it is possible to reduce the difference in the curvature radiusbetween the neighboring measurement positions in the circumferentialdirection of the outer circumferential end portion.

(4) End Surface Polishing Step

Next, end surface polishing is performed on the annular raw glass plate.In the same manner as the chamfering step, the end surface polishing isperformed by bringing the raw glass plate into contact with a polishingmeans while rotating the raw glass plate. Since no processing forgrinding or polishing the end surfaces is performed after the endsurface polishing, the end surface polishing plays an important role tosubstantially determine the final shape in the circumferentialdirection.

In the end surface polishing, a mass of a magnetic slurry is formed byholding the magnetic slurry along magnetic field lines, and this massand the inner circumferential end surface and the outer circumferentialend surface of the raw glass plate are brought into contact and movedrelative to each other, and thus the inner circumferential end surfaceand the outer circumferential end surface of the glass substrate arepolished. At this time, the side wall surfaces and the chamferedsurfaces can be polished simultaneously. The magnetic slurry contains amagnetic viscous fluid and microparticles of cerium oxide, zirconiumoxide, or the like as polishing abrasive particles. A fluid containing,for example, magnetic microparticles constituted by Fe and non-polar oilor polar oil is used as the magnetic viscous fluid. By polishing the endsurfaces, contamination by attached waste and the like, and damage orimpairment such as scratches on the end surfaces of the raw glass plateare eliminated, and therefore, it is possible to prevent thermalasperity and the deposition of ions such as sodium and potassium thatcauses corrosion. With the end surface polishing of this embodiment,extremely precise and high-quality processing is enabled compared with aconventional method for polishing end surfaces using a brush.Specifically, it is possible to significantly reduce the abnormality ofthe shape and to make surface roughness and waviness significantlysmall. In the case where brushing is performed, bristles are bent orbroken due to the front end of a brush coming into contact with andfollowing the work surface, and thus variability arises in the pressurethat is applied when the front end of the brush comes into contact withthe side wall surface or the chamfered surface. This causes partial deeppolishing streaks, for example, and thus the shape accuracy of the endsurface may be deteriorated. Also, in the same manner, a boundaryportion between the side wall surface and the chamfered surface isnon-uniformly ground in the circumferential direction, and thus theshape accuracy of the end surface may be deteriorated. It should benoted that the machining allowance in the end surface polishing step ofthe present invention can be greatly reduced compared with that in aconventional method using a brush, and the machining allowance can beset to, for example, 10 μm or less.

Here, the end surface polishing will be described in detail. FIGS. 3A to3C and FIG. 4 are drawings illustrating an example of a polishing methodin the end surface polishing of this embodiment.

An end surface polishing device 20 polishes the end surfaces of a glasssubstrate using a means for generating magnetism and a magnetic slurry.An outline of the end surface polishing device 20 will be described. Asshown in FIG. 3A, the device 20 includes, for example, a pair of magnets22 and 24, which are permanent magnets, a spacer 26, and a cylindricalpipe 28 made of a non-magnetic substance such as stainless steel. Themagnets 22 and 24 and the spacer 26 are incorporated in the pipe 28. Araw glass plate whose end surfaces are to be polished is held by aholder (not shown). As shown in FIG. 4, the pipe 28 is disposed near theouter circumferential end surface of the raw glass plate. A mass 30formed by the magnets 22 and 24 in this pipe 28 is brought into contactwith the outer circumferential end surface of the raw glass plate. Theholders (not shown) for holding the pipe 28 of the device 20 and the rawglass plate are mechanically connected to a driving motor (not shown).By rotating the pipe 28 and the holders to move the outercircumferential end surface of the raw glass plate and the mass 30relative to each other, it is possible to polish the outercircumferential end surface of the raw glass plate. It should be notedthat the end surfaces of the glass substrate and the mass 30 may bemoved relative to each other by fixing the pipes 28 and the holders androtating only the raw glass plate. It is sufficient to rotate the pipe28 at 500 to 5000 rpm, for example. Moreover, it is sufficient to rotatethe raw glass plate at 10 to 1000 rpm, for example. Although the pipe 28and the raw glass plate may be rotated in the down-cutting direction orthe up-cutting direction at the processing point, the down-cuttingdirection is more preferable because the polishing rate is low and theshape change very little. Moreover, it is preferable that the differencein a tangential velocity between the glass substrate and the magneticslurry is set to 800 m/min or less in the case where the rotation isperformed in each of the up-cutting direction and the down-cuttingdirection because it is possible to reduce the difference in the changein the shape in the circumferential direction between one surface andthe other surface (difference between surface A and surface B, whichwill be described later).

The end surface polishing will be described more specifically. Themagnet 22 and the magnet 24 are close to each other, serve as amagnetism generating means, and thus, as shown in FIG. 3B, form magneticfield lines 29 that extend from the magnet 22 to the magnet 24. Themagnetic field lines 29 extend so as to project outward from the centerbetween the magnets 22 and 24 and also extend in the thickness directionof the raw glass plate. The spacer 26 made of a non-magnetic substanceis provided between the magnets 22 and 24 in order to, for example, formthe mass 30 of magnetic slurry as shown in FIG. 3C around the outercircumference of the pipe 28.

Although it is sufficient that a magnetic flux density of the magnetismgenerating means is set so that the mass 30 of the magnetic slurry isformed, the magnetic flux density is preferably 0.1 to 10 tesla for thepurpose of performing end surface polishing efficiently.

It should be noted that in the example shown in FIGS. 3A to 3C and FIG.4, permanent magnets are used as the magnetism generating means, butelectromagnets can also be used. Also, the magnets 22 and 24 can befixed to the pipe 28 to ensure a constant distance between the endsurface on the N-pole side of the magnet 22 and the end surface on theS-pole side of the magnet 24 without the spacer 26.

Known glass substrate polishing abrasive particles such as cerium oxide,colloidal silica, zirconium oxide, alumina abrasive particles, ordiamond abrasive particles can be used as the polishing abrasiveparticles contained in the magnetic slurry. The polishing abrasiveparticles have the average particle diameter (D50) of, for example, 0.5to 10 μm. Using the polishing abrasive particles having a particlediameter within this range makes it possible to favorably polish theinner end surface of the raw glass plate. The polishing abrasiveparticles are contained in the magnetic slurry in an amount of, forexample, 1 to 20 vol %. Here, the average particle diameter (D50) meansa particle diameter at which the cumulative volume frequency calculatedin volume percentage in ascending order from small to large particlediameters reaches 50%.

(5) Precision Grinding Step

In a precision grinding step, the main surfaces of the annular raw glassplate are ground with a grindstone having fixed abrasive particles usinga double-side grinding device provided with a planetary gear mechanism.For example, a grinding pad to which diamond abrasive particles arefixed by a resin can be used as the grindstone having fixed abrasiveparticles. The double-side grinding device has a pair of upper and lowersurface plates (upper surface plate and lower surface plate) and theannular raw glass plate is held between the upper surface plate and thelower surface plate. The raw glass plate and the surface plates aremoved relative to each other by moving one or both of the upper surfaceplate and the lower surface plate, so that the two main surfaces of theraw glass plate can be ground.

(6) First Polishing (Main Surface Polishing) Step

Next, first polishing is performed on the ground main surfaces of theglass substrate. In the first polishing step, the double-side polishingdevice provided with a planetary gear mechanism is used. In thispolishing device, planar polishing pads having an annular shape as awhole are attached to the upper surface of the lower surface plate andthe bottom surface of the upper surface plate. During operation of theplanetary gear mechanism, the polishing pads are pressed against the rawglass plate mounted to a carrier and a polishing liquid is suppliedbetween the raw glass plate and the polishing pads. One example of amaterial for the polishing pad is urethane foam. A polishing liquidcontaining, for example, cerium oxide or zirconium oxide is used as thepolishing liquid.

(7) Chemical Strengthening Step

Next, the glass substrate on which the first polishing step has beenperformed is chemically strengthened.

For example, a mixed liquid of melts of potassium nitrate and sodiumsulfate can be used as a chemical strengthening liquid. The chemicalstrengthening is performed by immersing the raw glass plate in thechemical strengthening liquid.

(8) Second Polishing (Final Polishing) Step

Next, second polishing is performed on the raw glass plate that has beenchemically strengthened and sufficiently cleaned. In the secondpolishing, for example, a polishing device similar to the polishingdevice used in the first polishing is used. In this case, the secondpolishing differs from the first polishing in the type and size of looseabrasive particles and the hardness of a resin polisher.

For example, microparticles of colloidal silica (particle size: diameterof about 10 to 50 nm) are used as the loose abrasive particles to beused in the second polishing.

The polished raw glass plate is cleaned to provide a magnetic-disk glasssubstrate.

[Magnetic Disk]

A magnetic disk can be obtained as follows using the magnetic-disk glasssubstrate.

A magnetic disk has a configuration in which, for example, at least anattaching layer, a base layer, a magnetic layer (magnetic recordinglayer), a protecting layer and a lubricant layer are laminated on themain surface of the magnetic-disk glass substrate (referred to as merely“substrate” hereinafter) in this order from the side of the mainsurface.

For example, the substrate is introduced into a film deposition devicethat has been evacuated and the layers from the attaching layer to themagnetic layer are sequentially formed on the main surface of thesubstrate in an Ar atmosphere by a DC magnetron sputtering method. Forexample, CrTi can be used in the attaching layer and CrRu can be used inthe base layer. A CoPt based alloy can be used in the magnetic recordinglayer. Also, a CoPt-based alloy or a FePt-based alloy having an L₁₀ordered structure is formed to be the magnetic layer for thermallyassisted magnetic recording. After the film deposition as describedabove, by forming the protecting layer using C₂H₄ by, for example, a CVDmethod and subsequently performing nitriding processing that introducesnitrogen to the surface, a magnetic recording medium can be formed.Thereafter, by coating the protecting layer with perfluoropolyether(PFPE) by a dip coat method, the lubricant layer can be formed.

The produced magnetic disk is preferably incorporated in a magnetic-diskdrive device (hard disk drive (HDD)) serving as a magnetic recording andreproduction device provided with a magnetic head equipped with adynamic flying height (DFH) control mechanism and a spindle for fixingthe magnetic disk.

WORKING EXAMPLES AND COMPARATIVE EXAMPLES

In order to confirm the effect of the magnetic-disk glass substrateaccording to this embodiment, a magnetic-disk glass substrate having anominal diameter of 2.5 inches and a central value of the thickness of0.635 mm (having an outer diameter of 65 mm, an inner diameter of 20 mm,and a thickness of 0.635 mm) was produced, and a magnetic disk wasmanufactured.

Working Example 1

A magnetic-disk glass substrate of Working Example 1 was produced byperforming the steps of the above-described method for manufacturing amagnetic-disk glass substrate according to this embodiment in the givenorder.

Here, the pressing method was used in molding of a raw glass plate instep (1), and then the raw glass plate was subjected to rough grindingusing loose abrasive particles.

In the circular hole forming step in step (2), a circular hole wasformed in the center of the raw glass plate using a cylindrical drill.

In the chamfering in step (3), rough grinding for forming chamferedsurfaces was performed using a 400# diamond grindstone. Thereafter,final grinding was performed on the chamfered surfaces using a 2000#diamond grindstone. It should be noted that the shape of the chamferedportion was formed such that both θ1 and θ2 (see FIG. 1C) were 45degrees. Moreover, the distance D1 shown in FIG. 1C was 0.15 mm and thedistance D2 was 0.15 mm. It should be noted that all of the chamferedportions on the front side and back side of the inner circumferentialside and the outer circumferential side had the same shape.

In the end surface polishing in step (4), end surface polishing usingthe above-described magnetic slurry was performed. The magnetic slurryobtained by dispersing cerium oxide as polishing abrasive particles in amagnetic fluid obtained by dispersing Fe microparticles in non-polar oilwas used as the magnetic slurry. Permanent magnets were used as themagnets. Processing was performed in the up-cutting direction. It shouldbe noted that the machining allowance of the chamfered surfaces was 10μm in step (4).

In the grinding using fixed abrasive particles in step (5), grinding wasperformed using a grinding device in which a grinding pad to whichdiamond abrasive particles were fixed by a resin and that was to be usedas a grindstone having fixed abrasive particles was attached to asurface plate.

In the first polishing in step (6), a polishing liquid containing ceriumoxide abrasive particles was used, and a hard urethane pad was used asthe polishing pad.

In the chemical strengthening in step (7), the raw glass plate wasimmersed in a mixed liquid of melts of potassium nitrate and sodiumnitrate as a chemical strengthening liquid.

In the second polishing in step (8), a polishing liquid containingcolloidal silica microparticles as abrasives was used. Thereafter, theraw glass plate was cleaned, and a magnetic-disk glass substrate wasobtained.

Working Example 2

A magnetic-disk glass substrate was manufactured in the same manner asin Working Example 1, except that the machining allowance was set to 8μm in the end surface polishing in step (4).

Working Example 3

A magnetic-disk glass substrate was manufactured in the same manner asin Working Example 2, except that rough grinding was performed using a500# diamond grindstone and final grinding was performed using a 3000#diamond grindstone in the chamfering step in step (3), and that themachining allowance was set to 8 μm in the end surface polishing in step(4).

Working Example 4

A magnetic-disk glass substrate was manufactured in the same manner asin Working Example 3, except that the machining allowance was set to 5μm in the end surface polishing in step (4).

Working Example 5

A magnetic-disk glass substrate was manufactured in the same manner asin Working Example 3, except that the magnets and the raw glass platewere rotated in the down-cutting direction at the processing point inthe end surface polishing in step (4).

Working Example 6

A magnetic-disk glass substrate was manufactured in the same manner asin Working Example 5, except that the machining allowance was set to 8μm in the end surface polishing in step (4).

Working Example 7

A magnetic-disk glass substrate was manufactured in the same manner asin Working Example 5, except that the machining allowance was set to 4μm in the end surface polishing in step (4).

Comparative Example 1

On the other hand, in Comparative Example 1, rough grinding for formingchamfered surfaces was performed using a 400# diamond grindstone in thechamfering in step (3). It should be noted that final grinding was notperformed on the comparative example 1.

In addition, in the end surface polishing in step (4), the end surfacesof the raw glass plate were polished with a polishing brush using ceriumoxide as loose abrasive particles. The machining allowance of thechamfered surfaces was 50 μm in step (4).

Comparative Example 2

A magnetic-disk glass substrate was manufactured in the same manner asin, Comparative Example 1, except that rough grinding was performedusing a 500# diamond grindstone and final grinding was performed using a3000# diamond grindstone in the chamfering in step (3), and that themachining allowance of the chamfered surfaces was set to 30 μm in thebrushing in step (4).

Next, in each of the working examples and comparative examples,measurement points were provided every 30 degrees in the circumferentialdirection of the main surface with reference to the center of the mainsurface, and the curvature radius of the shape of the portion betweenthe side wall surface and the chamfered surface was determined at eachmeasurement point. It should be noted that the measurement was performedon twenty-four points in total, consisting of twelve points on the frontsurface side and twelve points on the back surface side, in the outercircumferential end portion of each glass substrate selected from theworking examples and comparative examples. The differences in thecurvature radius between the neighboring measurement points of thetwelve points on the front surface side (twelve pieces of data) and thedifferences in the curvature radius between the neighboring measurementpoints of the twelve points on the back surface side (twelve pieces ofdata) were determined, and the maximum value among the twenty-fourpieces of data in total was considered as the maximum value of thecurvature radius of the corresponding working example or comparativeexample. Moreover, the second curvature radius of the portion betweenthe chamfered surface and the main surface was determined in the samemanner, which will be described later. Table 1 shows a portion of themeasurement data (Comparative Examples 1 and 2 and Working Examples 1and 3). In Table 1, the front surface and the back surface of the glasssubstrate to be measured are respectively represented as surface A andsurface B. Moreover, in Table 1, the difference in the curvature radiusof, for example, “0 to 30°” means an absolute value of the differencebetween the curvature radius at the measurement point at 0 degree andthe curvature radius at the measurement point at 30 degrees.Furthermore, for example, the position at 30 degrees on surface B wasdisposed on the back side of the position at 30 degrees on surface A.

TABLE 1 Difference in curvature radius (mm) Work. Ex. 1 Work. Ex. 2Comp. Ex. 1 Comp. Ex. 2 Sur- Sur- Sur- Sur- Sur- Sur- Sur- Sur- faceface face face face face face face A B A B A B A B 0 to 30° 0.005 0.0090.001 0.004 0.010 0.001 0.010 0.001 30 to 60° 0.004 0.007 0.001 0.0040.013 0.001 0.003 0.001 60 to 90° 0.005 0.007 0.002 0.005 0.020 0.0070.001 0.002 90 to 120° 0.003 0.008 0.003 0.003 0.004 0.007 0.012 0.003120 to 150° 0.003 0.007 0.005 0.001 0.013 0.001 0.009 0.005 150 to 180°0.003 0.006 0.005 0.004 0.005 0.015 0.000 0.007 180 to 210° 0.005 0.0060.003 0.003 0.010 0.015 0.003 0.011 210 to 240° 0.005 0.006 0.002 0.0040.012 0.016 0.010 0.010 240 to 270° 0.004 0.007 0.001 0.003 0.011 0.0160.009 0.001 270 to 300° 0.005 0.009 0.004 0.002 0.009 0.002 0.011 0.004300 to 330° 0.005 0.009 0.002 0.001 0.003 0.002 0.004 0.002 330 to 360°0.002 0.010 0.003 0.002 0.002 0.002 0.004 0.003 Maximum 0.010 0.0050.020 0.012 value of difference

A magnetic disk was produced by forming a magnetic layer on the obtainedmagnetic-disk glass substrate. Thereafter, fluttering was evaluated foreach magnetic disk of the working examples and comparative examplesusing a laser Doppler vibrometer. To evaluate fluttering, first, themagnetic disk was attached to the spindle of a hard disk drive (HDD)having a rotation rate of 7200 rpm, and the main surface of the rotatingmagnetic disk was irradiated with a laser beam from a laser Dopplervibrometer. Next, the laser Doppler vibrometer received the laser beamreflected by the magnetic disk, and thus the vibration value in thethickness direction of the magnetic disk was obtained.

The following is more specific description.

In the measurement of the fluttering characteristic value, a magneticdisk was attached to the spindle of a 2.5-inch type HDD and was rotated,and the main surface of the rotating magnetic disk was irradiated with alaser beam from a laser Doppler vibrometer. It should be noted that theHDD was properly equipped with a cover so as not to be affected byoutside air, and the cover of the HDD was provided with a hole for laserbeam irradiation. Next, the laser Doppler vibrometer received the laserbeam reflected by the magnetic disk, and thus the amount of vibration inthe thickness direction of the magnetic disk was measured as afluttering characteristic value. In this case, the flutteringcharacteristic values were measured under the following conditions.

-   -   Environment for HDD and measurement system: The temperature was        kept at 25° C. in a constant temperature and humidity chamber.    -   Rotation rate of magnetic disk: 7200 rpm    -   Laser beam irradiation position: Position 31 mm apart from the        center (1.5 mm apart from the outer circumferential end) of a        magnetic disk in the radial direction    -   Minimum value of diameter of inner wall of disk-attaching        portion in HDD housing: 65.880 mm

[Evaluation Criterion]

The results of evaluation of the measured fluttering characteristicvalues were divided to four levels, Levels 1 to 4, in descending orderof favorability (that is, in increasing order of the flutteringcharacteristic value). Levels 1 and 2 are acceptable for practicalpurposes. Tables 2 and 3 show the results.

Level 1: 20 nm or less

Level 2: more than 20 nm to 30 nm or less

Level 3: more than 30 nm to 40 nm or less

Level 4: more than 40 nm

TABLE 2 Maximum value of difference in Evaluation curvature radius (mm)of fluttering Comp. Ex. 1 0.020 Level 4 Comp. Ex. 2 0.012 Level 4 Work.Ex. 1 0.010 Level 2 Work. Ex. 2 0.008 Level 2 Work. Ex. 3 0.005 Level 1Work. Ex. 4 0.003 Level 1

TABLE 3 Maximum value of difference in Evaluation second curvatureradius (mm) of fluttering Work. Ex. 1 0.006 Level 2 Work. Ex. 5 0.005Level 2 Work. Ex. 6 0.004 Level 1 Work. Ex. 7 0.002 Level 1

As shown in Table 2, in the case where the difference in the curvatureradius between the neighboring measurement points was 0.01 mm or less,favorable evaluation was obtained. It is found from these results thatthe reduction of the change in the shape of the outer circumferentialend surface in the circumferential direction of the glass substratemakes it possible to further reduce fluttering at a high rotation speed.Moreover, as shown in Table 2, in the case where the difference in thecurvature radius between the neighboring measurement points was 0.005 mmor less, even more favorable evaluation was obtained compared with thecase where the difference was 0.01 mm or less.

It should be noted that in Comparative Examples 1 and 2, when thecurvature radius was measured every 40 degrees in the circumferentialdirection and the maximum value of the difference in the curvatureradius between the neighboring measurement points was determined in thesame manner as described above, the maximum values of both comparativeexamples were 0.01 mm or less. Also, in the case where the curvatureradius was measured every 60 degrees, the maximum values of bothcomparative examples were 0.01 mm or less. Accordingly, it was revealedthat the measurement every 30 degrees was important. That is, it isassumed that the measurement every 30 degrees made it possible to detectchanges in the shape of the end portions during a short cycle and thus acorrelation with fluttering was obtained. With the present invention, byperforming the end surface polishing using a magnetic slurry, it ispossible to suppress abnormality of the shape of the end portions in thecircumferential direction that occurs in the case where the end portionsare polished using a brush, thus making it possible to reducefluttering.

Table 3 shows the evaluation results in the case where the differencesin the curvature radius between the neighboring measurement points are0.01 mm or less and the maximum values of the differences in the secondcurvature radius between the neighboring measurement points aredifferent. It should be noted that fluttering was evaluated in the samemanner as described above, except that a 2.5-inch HDD having a rotationrate of 10000 rpm was used. As shown in Table 3, in the case where thedifference in the second curvature radius between the neighboringmeasurement points was 0.004 mm or less, more favorable evaluation wasobtained.

[Evaluation with Different Glass Composition]

Next, in order to further confirm the effect of the method formanufacturing a magnetic-disk glass substrate according to thisembodiment, a 2.5-inch magnetic disk was produced from a magnetic-diskglass substrate that had glass composition 2 different from theabove-described glass composition 1 (Working Example 8). The method forproducing a magnetic-disk glass substrate is the same as the case whereglass has glass composition 1 (that is, the above-described steps (1) to(8)). It should be noted that glass composition 2 is preferably for acomposition of glass to be used in a magnetic-disk glass substrate to beused in a magnetic disk for energy-assisted magnetic recording.

In Working Example 8, at measurement points provided every 30 degrees inthe circumferential direction of the main surface with reference to thecenter of the main surface, the curvature radius of the shape of theportion between the side wall surface and the chamfered surface wasdetermined at each measurement point. As in Working Example 1, themaximum value of the difference in the curvature radius between theneighboring measurement points was 0.01 mm.

Next, when fluttering of Working Example 8 was evaluated, a favorableresult was obtained as in Working Example 1.

Next, in order to further confirm the effect of the method formanufacturing a magnetic-disk glass substrate according to thisembodiment, 2.5-inch magnetic disks were produced from magnetic-diskglass substrates having different thicknesses.

[Evaluation with different thickness]

Magnetic-disk glass substrates were manufactured in the same manner asin Working Example 1 and Comparative Example 1, except that a centralvalue of thickness was 0.500 mm, and magnetic disks were obtained byforming films on these magnetic-disk glass substrates (Working Example 9and Comparative Example 3, respectively).

Moreover, magnetic-disk glass substrates were manufactured in the samemanner as in Working Example 1 and Comparative Example 1, except that acentral value of thickness was 0.800 mm, and magnetic disks wereobtained by forming films on these magnetic-disk glass substrates(Working Example 10 and Comparative Example 4, respectively).

Furthermore, magnetic-disk glass substrates were manufactured in thesame manner as in Working Example 1 and Comparative Example 1, exceptthat a central value of thickness was 1.000 mm, and magnetic disks wereobtained by forming films on these magnetic-disk glass substrates(Working Example 11 and Comparative Example 5, respectively).

When the maximum value of the curvature radius of the outercircumferential end surface was evaluated, those of Working Examples 9,10 and 11 were the same as that of Working Example 1, and those ofComparative Examples 3, 4 and 5 were the same as that of ComparativeExample 1.

Fluttering of each magnetic disk of Working Examples 1, 9, 10 and 11 andComparative Examples 1, 3, 4 and 5 was evaluated in the same manner asdescribed above, except that a 2.5 inch HDD having a rotation rate of5400 rpm was used. The improved widths in the fluttering characteristicvalues between working examples and comparative examples having the samethickness (values obtained by subtracting the fluttering characteristicvalue of the working example from the fluttering characteristic value ofthe comparative example) that were determined are as follows.

-   -   Improved width when thickness was 1.000 mm: 2.6 nm    -   Improved width when thickness was 0.800 mm: 5.0 nm    -   Improved width when thickness was 0.635 mm: 10.0 nm    -   Improved width when thickness was 0.500 mm: 20.5 nm

It is confirmed from the above-described results that the presentinvention exhibits a particularly large improved effect in the casewhere the thickness is 0.635 mm or less.

Next, the relationship between fluttering and the difference betweensurface A and surface B in the difference in curvature radius betweenthe neighboring points was examined. Specifically, the average values ofthe difference in curvature radius between the neighboring points ofsurface A and surface B were determined, and the difference betweensurface A and surface B (ΔR) was obtained in an absolute value bydetermining the difference between the average value in surface A andthe average value in surface B. ΔR was changed by controlling thetangential velocity at the processing point during the end surfacepolishing based on the manufacturing conditions of Working Example 1.The difference in the tangential velocity was set to 800 m/min or lessin Working Examples 12 and 13, and to more than 800 m/min in WorkingExamples 14 and 15.

Moreover, fluttering was evaluated in the same manner as describedabove, except that a 2.5 inch HDD having a rotation rate of 15000 rpmwas used. It should be noted that this evaluation was purposelyperformed under very severe conditions, and therefore, there is noproblem in practical terms even at Level 3.

As a result of evaluation, it was found that setting ΔR to 0.003 mm orless makes the fluttering characteristics when the magnetic disk isrotated at an ultrahigh rotation speed even more favorable. It was foundfrom this fact that it is important that surface A and surface B have asimilar value of the difference in curvature radius between theneighboring points in the circumferential direction when the magneticdisk is rotated at an ultrahigh rotation speed.

TABLE 4 Evaluation ΔR (mm) of fluttering Work. Ex. 12 0.002 Level 2Work. Ex. 13 0.003 Level 2 Work. Ex. 14 0.004 Level 3 Work. Ex. 15 0.005Level 3

While the magnetic-disk glass substrate and magnetic disk according tothe present invention has been described in detail, the presentinvention is not limited to the above-described embodiment, and it willbe appreciated that various improvements and modifications can be madewithout departing from the concept of the present invention.

The invention claimed is:
 1. A doughnut-shaped magnetic-disk glasssubstrate having a circular hole provided in the center, thedoughnut-shaped magnetic-disk glass substrate comprising: a pair of mainsurfaces; and an outer circumferential end surface and an innercircumferential end surface each including a side wall surface and achamfered surface that is formed between each main surface and the sidewall surface, a measurement point being provided on the outercircumferential end surface every 30 degrees in the circumferentialdirection with reference to a center of the glass substrate, and when acurvature radius of a shape of a portion between the side wall surfaceand the chamfered surface is determined at each measurement point, adifference in the curvature radius between neighboring measurementpoints being 0.01 mm or less.
 2. The magnetic-disk glass substrateaccording to claim 1, wherein the difference in the curvature radiusbetween neighboring measurement points is 0.005 mm or less.
 3. Themagnetic-disk glass substrate according to claim 2, wherein ameasurement point is provided every 30 degrees in the circumferentialdirection with reference to the center of the glass substrate, and whena curvature radius of a shape of a portion between the main surface andthe chamfered surface is determined at each measurement point as asecond curvature radius, a difference in the second curvature radiusbetween neighboring measurement points is 0.004 mm or less.
 4. Themagnetic-disk glass substrate according to claim 3, wherein the glasssubstrate has a thickness of 0.635 mm or less.
 5. A magnetic disk inwhich at least a magnetic layer is formed on the magnetic-disk glasssubstrate according to claim
 4. 6. A magnetic disk in which at least amagnetic layer is formed on the magnetic-disk glass substrate accordingto claim
 3. 7. The magnetic-disk glass substrate according to claim 2,wherein the glass substrate has a thickness of 0.635 mm or less.
 8. Amagnetic disk in which at least a magnetic layer is formed on themagnetic-disk glass substrate according to claim
 7. 9. A magnetic diskin which at least a magnetic layer is formed on the magnetic-disk glasssubstrate according to claim
 2. 10. The magnetic-disk glass substrateaccording to claim 1, wherein a measurement point is provided every 30degrees in the circumferential direction with reference to the center ofthe glass substrate, and when a curvature radius of a shape of a portionbetween the main surface and the chamfered surface is determined at eachmeasurement point as a second curvature radius, a difference in thesecond curvature radius between neighboring measurement points is 0.004mm or less.
 11. The magnetic-disk glass substrate according to claim 10,wherein the glass substrate has a thickness of 0.635 mm or less.
 12. Amagnetic disk in which at least a magnetic layer is formed on themagnetic-disk glass substrate according to claim
 11. 13. A magnetic diskin which at least a magnetic layer is formed on the magnetic-disk glasssubstrate according to claim
 10. 14. The magnetic-disk glass substrateaccording to claim 1, wherein the glass substrate has a thickness of0.635 mm or less.
 15. A magnetic disk in which at least a magnetic layeris formed on the magnetic-disk glass substrate according to claim 14.16. A magnetic disk in which at least a magnetic layer is formed on themagnetic-disk glass substrate according to claim 1.